Legal

Runs on anything

OpenRA - RTS - An open source recreation of the Tiberian Dawn engine. All games are implemented as mods, making it highly extensible. Currently has mods for Command and Conquer: Tiberian Dawn, Command and Conquer: Red Alert and Dune 2000.

Red Alert 2 and Tiberian Sun are planned to be implemented in the future, though it will likely be months until we see a release.
http://www.openra.net/

Command and Conquer: Tiberian Sun - RTS - An RTS made by Westwood Studios as a sequel to the incredibly popular Tiberian Sun. Features an earth in its dying days, with two factions, the Global Defence Initiative and the Brotherhood of Nod battling over a solution to the crisis. Slow-paced, very ambinet. Has a Twisted Insurrection total conversion mod which tells the story of what would happen if Nod won the previous war rather than GDI.

http://cnc-comm.com/community/index.php?action=downloads;sa=view;down=41
http://ti.ppmsite.com/

Needs a decent PC

Mari0 - Platformer - It's the original Super Mario Bros. with a portal gun, customizable appearances and multiplayer.

http://stabyourself.net/mari0/

Needs a good PC

Renegade X - FPS/RTS - A remake of a 2003 game called Command and Conquer: Renegade made in the UDK. Features fast-paced, team-based gameplay where each team must collaborate in order to bring down the other's base.

http://renegade-x.com/

Less than legal

Ask Kriemhild Gretchen about where to legitimately purchase these

Command and Conquer: Red Alert 2: Yuri's Revenge - RTS - The expansion to the sequel of Red Alert. The enigmatic psychic Yuri's servants travel back in time to rescue him from being killed after his rebellion in the Third Great World War. A total conversion mod called Mental Omega has been made, which massively improves the game and implements three different subfactions for each faction, leading to a total of nine factions overall.

http://mentalomega.com/

Civilization 5 - Turn-based strategy - A turn-based strategy game where you lead civilizations, either to glory, infamy or something more.

Terraria - Platformer - A Minecraft-inspired platformer where you mine, build and kill things.